Submission #2151028

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> P;
typedef pair<int,P> P1;
typedef pair<P,P> P2;
#define pb push_back
#define mp make_pair
#define eps 1e-7
#define INF 1000000000
#define mod 1000000007
#define fi first
#define sc second
#define rep(i,x) for(int i=0;i<x;i++)
#define repn(i,x) for(int i=1;i<=x;i++)
#define SORT(x) sort(x.begin(),x.end())
#define ERASE(x) x.erase(unique(x.begin(),x.end()),x.end())
#define POSL(x,v) (lower_bound(x.begin(),x.end(),v)-x.begin())
#define POSU(x,v) (upper_bound(x.begin(),x.end(),v)-x.begin())
int n;
vector<int>edge[1005];
P DFS(int v,int u,int d){
	P ret = mp(d,v);
	rep(i,edge[v].size()){
		if(edge[v][i]==u) continue;
		ret=max(ret,DFS(edge[v][i],v,d+1));
	}
	return ret;
}
int dist[1005][2];
void DFS2(int v,int u,int d,int ty){
	dist[v][ty] = d;
	rep(i,edge[v].size()){
		if(edge[v][i]==u) continue;
		DFS2(edge[v][i],v,d+1,ty);
	}
}
int must,u,v;
bool ok(int x,int val){
	int D = dist[x][0];
	if( (D+abs(val))/2 > (dist[v][0]+1)/2 ) return false;
	D = dist[x][1];
	if( (D+abs(val-must))/2 > (dist[v][0]+1)/2 ) return false;
	return true;
}
ll dfs(int x,int y,int val){
	if(x == v){
		if(must == val) return 1LL;
		else return 0LL;
	}
	ll ret = 1;
	rep(i,edge[x].size()){
		if(edge[x][i] == y) continue;
		ll add = 0;
		if(ok(edge[x][i],val+1)){
			add += dfs(edge[x][i],x,val+1);
		}
		if(ok(edge[x][i],val-1)){
			add += dfs(edge[x][i],x,val-1);
		}
		add %= mod;
		ret = ret*add%mod;
	}
	return ret;
}
int main(){
	cin>>n;
	rep(i,n-1){
		int a,b; cin>>a>>b;
		edge[a].pb(b);
		edge[b].pb(a);
	}
	P p = DFS(1,-1,0);
	P q = DFS(p.sc,-1,0);
	u = p.sc, v = q.sc;
	DFS2(u,-1,0,0);
	DFS2(v,-1,0,1);
	if(q.fi%2 == 0){
		must = 0;
		cout << dfs(u,-1,0) << endl;
	}
	else{
		ll ans = 0;
		must = 1;
		ans += dfs(u,-1,0);
		must = -1;
		ans += dfs(u,-1,0);
		cout << ans%mod << endl;
	}
}

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